あいし～きゅ～ ボクっ娘なんて呼ばないでっ

Main character had a close childhood friend, but lost connection with him due to catching a serious disease and undergoing treatment in a distant sanatorium. He finally returns only to find that his close childhood friend is actually a girl! She is considered a boy by everyone else, but when they remain alone passion can't be concealed.

[From vndbreview]